I’ve a raspberry with raspbian Buster (updated from stretch) and HA installed. My wifi has from a few days a strange problem. After a power on… I can connectly via ssh with no problem. But After some seconds… My raspberry disappear from my lan! I can’t access in HA and can’t access with SSH. Ip is assigned always the same. Using a network scanner the raspberry is not present. When I try to power on the raspberry with hdmi and monitor, mouse and keyboard to see eventually errors, i can’t replicate the problem 
hardware issues? HA related?
I have no experience with buster yet, but with older raspi OS versions you always had to make sure to disable the wifi power management.
iwconfig (hope that command still exists with buster) should give you a hint if power management is on.
I’ll try. With older stretch version i’ve never problem with wifi… Never disabled power management.